The present invention relates to demodulation of radio signals modulated with a new, spectrally-efficient type of modulation while compensating for time dispersion caused by multipath propagation in the land-mobile radio environment.
The European digital cellular system known as GSM utilizes a modulation technique known as Gaussian Minimum Shift Keying (GMSK), in which successive binary bits are modulated alternately onto a cosine carrier and a sine carrier respectively while maintaining a constant signal amplitude. A related modulation called Offset-shaped Quadrature Phase Shift Keying (xe2x80x9cOffset QPSKxe2x80x9d or xe2x80x9cOQPSKxe2x80x9d) provides a very similar signal but does not maintain a constant amplitude. Nevertheless, the similarity between GMSK and OQPSK waveforms is sufficient enough to make it possible for an OQPSK receiver to efficiently demodulate a GMSK signal and vice versa. Cellular telephones conforming to the GSM standard have been marketed and sold throughout the world by L. M. Ericsson since about 1990, and since about 1993 in the U.S.A. where the standard is known as PCS1900. L. M. Ericsson is the Swedish parent company of Ericsson Inc., the current assignee of this application. Ericsson GSM telephones utilize derotation of the received GMSK signal samples by successive multiples of 90 degrees. However, the information symbols that are derotated are symbols representing single binary bits that can take on levels of only +1 or xe2x88x921, and not the multi-level signals disclosed in this application and its parent application. GSM telephones also employ channel estimation by correlating received signal samples with a known synchronization word (xe2x80x9csyncwordxe2x80x9d) and then using the channel estimates in a Viterbi processor to demodulate the received signal while compensating for inter-symbol interference (ISI). Indeed, the Viterbi processor is a well known form of equalizer used to compensate for multipath propagation and other causes of ISI, and is described for example in U.S. Pat. Nos.:
5,093,848;
5,136,616;
5,331,666;
5,335,250;
5,577,068;
5,568,518;
5,615,231;
5,557,645; and
5,619,553
which are all hereby incorporated by reference herein.
A related inventive modulation and decoding method therefor is disclosed in U.S. patent application Ser. No. 08/769,263 to P. Dent, filed Dec. 18, 1996 and entitled xe2x80x9cSpectrally Efficient Modulation Using Overlapped GMSKxe2x80x9d, which is also hereby incorporated by reference herein. In overlapped GMSK modulation, the number of data bits transmitted in a given instant and within a given bandwidth is doubled by combining two GMSK signals with a 90 degree relative phase rotation to reduce interference therebetween. In the parent to this application (i.e., U.S. patent application Ser. No. 08/662,940), it is disclosed that two GMSK signals may alternatively be combined with relative amplitude ratio 1:0.5 in order to produce an inventive offset-16QAM modulation.
In the prior art, the number of metric computations performed in a Viterbi equalizer to demodulate each symbol from an alphabet of M symbols is equal to M to the power L, where L is the number of symbols of ISI, that is the number of symbols which affect each received signal sample. In the GSM system, a GMSK symbol may be regarded as having a duration of two bits, but thanks to the one bit or half-symbol offset between the cosine and sine channels, a half-symbol may be regarded as being only one bit duration. By Viterbi processing to demodulate one half-symbol at a time, the GSM telephones therefore reduce complexity by having a sub-alphabet size of only two possible bit values, that is M=2 for GSM equalizers sold by Ericsson.
It is therefore an object of the present invention to provide communications techniques and apparatus that reduce the number of required metric computations even when higher order constellations such as 16-QAM are used.
This application discloses optimum methods to decode the inventive modulation described in the above-referenced U.S. patent application Ser. No. 08/662,940. More particularly, the invention disclosed herein enables the metric savings to be obtained even when higher order constellations such as 16-QAM are used, by employing the teachings of the parent application to offset the instants at which cosine and sine modulation are applied by half of a 16-QAM symbol duration to produce the inventive 16-OQAM modulation. By processing only a cosine symbol or a sine symbol at a time, the sub-alphabet size is reduced from sixteen possible symbols to four possible symbols, that is, M=4 rather than 16, giving a much lower equalizer complexity than that which is disclosed in the prior art.
In accordance with one aspect of the present invention, an inventive transmitter encodes a number 2N (e.g., 2N=4) data bits by using N (e.g., two) data bits to select one of two to the power N (e.g., four) levels of a cosine wave and the other N data bits to select one of two to the power N levels of a sine wave. In contrast to prior art 16QAM, the inventive modulation attains the cosine wave levels at instants between the instants that the sine wave attains its modulation levels, that is, at instants offset by half a 2N-bit (e.g., 4-bit) symbol interval, the modulation thus being known as Offset QAM or OQAM. A receiver according to the invention receives the OQAM signal and amplifies, filters and digitizes the received signal at a sampling rate of preferably only two samples per 2N-bit symbol interval (i.e., one sample per N-bit half-symbol interval). Successive N-bit half-symbols comprise information modulated alternately on a cosine and a sine carrier wave, that is, successive half-symbols are rotated by 90 degrees. In one embodiment of the inventive modulation, this rotation successively has the values 0,90,0,90,0,90 . . . and so on, while in an alternative embodiment of the inventive modulation, the successive rotation takes on the values 0,90,180,270,0,90,180,270 . . . and so on, such that successive cosine wave symbols are alternately inverted (0,180,0,180 . . . ) while successive sine wave symbols are likewise alternately inverted (90,270,90,270 . . . ). This alternate inversion of successive cosine and successive sine wave symbols does not change the modulation in principle, and merely requires that the successive inversion be corrected by inverting alternate cosine and sine symbols either before modulation (by use of preceding), or alternatively, after demodulation.
The inventive receiver may optionally remove the successive rotation through 0,90,180,270 . . . degrees by applying successive derotations of successive digitized samples by like amounts. Successive derotations of successive digitized samples are easy to implement by simply swapping the real and imaginary parts of the complex samples and manipulating their signs appropriately. After this derotation, the inventive receiver performs a correlation of the derotated samples with a set of known sync half-symbols to establish a time synchronization marker for demodulating unknown N-bit half-symbols. The sync half-symbols may furthermore be chosen to comprise only two of the 2N possible amplitudes of the sine or cosine wave, such as the two greatest amplitudes of opposite sign, such that the sync symbols are binary symbols, facilitating correlation.
The sync correlations determine a set of channel coefficients describing the dependence of each digitized sample on one or more unknown half-symbols. Dependence of a digitized sample on more than one successive half-symbol may occur for example due to time dispersion or multipath echos in the propagation path, inter-symbol interference (ISI) caused by transmit or receiver filtering, or by the receiver digitizing signal samples at a sampling instant other than that at which the half-symbols are expected to attain their nominal sine or cosine values.
The channel estimates computed using known half-symbols are then used to predict the expected received sample values for all different possible sequences of successive unknown half-symbols that are to be decoded. For example, where N=2, if channel estimation determines that a received sample depends on 3 successive half-symbols, there are 4 cubed=64 possible sequences of 3 successive two-bit half-symbols, and thus 64 possible expected values of which 32 are simply the negatives of the other 32.
Continuing with the example in which N=2, received samples are then compared with all 64 possible expected values and a measure of disagreement or error metric computed. A Viterbi Maximum Likelihood Sequence Estimator (xe2x80x9cViterbi MLSExe2x80x9d, or xe2x80x9cMLSExe2x80x9d) is preferably used to accumulate error metrics to determine that sequence, among all possible sequences, that produces the lowest cumulative error metric. Automatic phase correction and amplitude scaling during Viterbi processing may take place using fast automatic frequency control (AFC) and automatic gain control (AGC) algorithms. The number of metric computations using this inventive 16-OQAM modulation is equal to 4 to the power L, where L is the number of half-symbols on which each digitized sample depends. This is contrasted with 16 to the power L for prior art 16-QAM. The invention may be obviously extended to higher order constellations of OQAM other than 16-OQAM.
In other aspects of the invention, dual mode transmitters and receivers are disclosed that enable different types of modulation to be alternatively utilized. In some embodiments, a communication burst includes unknown as well as known (sync) information symbols, wherein the known sync symbols communicated between the transmitter and the receiver convey information permitting the receiver to determine the type of modulation utilized for transmitting the unknown information symbols. In some embodiments, the known sync information symbols are transmitted by means of only one type of modulation (e.g., GMSK) regardless of the type of modulation applied for transmitting the unknown information symbols.